Known tube cutting assemblies of this type, often referred to as slitters by people skilled in the art, normally comprise a pair of opposite rotatable cutting wheels which are brought into engagement with the tube wall and co-operate to cut or sever the rotating tube. A first cutting wheel is disposed inside the tube and a second cutting wheel is disposed outside the tube. When the tube is to be cut, the outer cutting wheel is brought into engagement with the outer surface of the tube and its cutting edge cuts through the tube wall against the cutting edge of the inner cutting wheel. An example of such a tube cutting assembly is disclosed in U.S. Pat. No. 5,636,541 and its counterpart EP-A-749,787 which is hereby incorporated by reference (see in particular FIGS. 13 and 14).
Under normal circumstances these known tube cutting assemblies work very well, but in certain cutting operations the end of the cut tube is deformed in an undesirable manner. This is illustrated in FIG. 1 which shows a cut tube seen from the end. Such undesirable deformations of the tube wall at the end sometimes occur on thin-walled tubes when the two cutting wheels are brought into engagement with the tube wall at high speed. The same difficulties may arise when the rotational speeds of the tube and the cutting wheels are not synchronized. The problem is particularly common when cutting helically-wound lock-seam tubes, since the helical lock seam causes “bumps” at the cutting location between the cutting wheels which in turn may lead to deformations of the end of the cut tube.
The deformations at the end of the cut tube make it hard to connect various tubular components to the tube which are inserted in the tube from the end. When mounting ventilation duct systems including helically-wound lock-seam tubes of sheet metal, fittings like bends, T pieces, reducers and other coupling members are connected to the ventilation ducts. If these fittings do not fit the ducts, the fitter either has to eliminate the end deformations manually or, if it comes to the worst, discard the duct as scrap.